Energy saving method in wireless network

ABSTRACT

Provided is a method of minimizing power consumed by each node in a wireless network when communicating with neighboring nodes. 
     Unlike a conventional synchronous media access control (MAC), the present invention suggests an asynchronous MAC. Thus, the active duration of a node extends only when transmitting a large quantity of data and the extended active duration returns to the minimum active duration again after completing the data transmission.

CROSS-REFERENCE TO RELATED PATENT APPLICATION

This application claims the benefit of Korean Patent Application No.10-2007-0128228, filed on Dec. 11, 2007, and Korean Patent ApplicationNo. 10-2008-0065595, filed on Jul. 7, 2008, in the Korean IntellectualProperty Office, the disclosure of which is incorporated herein in itsentirety by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a method of minimizing power consumedby each node in a wireless network, and in particular, in a meshnetwork.

The present invention is supported by the Information Technology (IT)Research & Development (R&D) program of the Ministry of Information andCommunication (MIC) and the Institute for Information TechnologyAdvancement (IITA) [2005-S-038-03, Development of UHF RF-ID andUbiquitous Networking Technology].

2. Description of the Related Art

Since sensor nodes in a wireless sensor network environment operatebased on limited energy, much research has been conducted on low poweroperation thereof. In particular, since a general sensor node operatingusing batteries does not support low power, services provided by thesensor node can be blocked. Accordingly, research on a low powerprotocol has been actively conducted.

In zigbee MAC, a synchronous communicating method generally usedincludes adjusting the communication time of each node. A system usingsuch method is formed of a plurality of nodes which can be classifiedinto a general node, a router node, and a sink node. The general node isthe lowest portion for performing time synchronization. The router nodereceives a beacon message from a sink node or another router node andtransmits the beacon message to a lower router or a general node,thereby matching the time of each node. The sink node mainly performstime synchronization in a network and transmits synchronizationinformation, such as policy and time, in a network to a general node ora router node. Such method hardly adjusts synchronization and supportslow power.

SUMMARY OF THE INVENTION

The present invention provides an energy saving method so that a nodecan minimize power consumption by increasing activation time when datato be transmitted by the node exists in a mesh network and by minimizingactivation time when data to be transmitted by the node does not existin mesh network.

According to an operation of the mesh network when the active durationof the node is minimized, the active duration extends only whencommunicating a large quantity of data so as to save and minimize thepower consumption and the extended active duration returns to theminimum active duration after completing data transmission, therebyflexibly varying the active duration.

According to an aspect of the present invention, there is provided anenergy saving method performed in a node in a wireless network, themethod including: receiving a wakeup notification from a destinationnode by a source node having data to be transmitted, wherein the sourcenode is one of a plurality of nodes sending a wakeup notification in aregular interval and a wakeup notification interval is equal to a wakeupinterval formed of an active duration and an inactive duration; andtransmitting data within an active duration of the destination node whenthe active duration estimated based on the wakeup notification isgreater than a data size.

According to another aspect of the present invention, there is providedan energy saving method performed in node in a wireless network, themethod including: requesting extension of an active duration toneighboring nodes by a source node having data to be transmitted,wherein the source node is one of a plurality of nodes sending a wakeupnotification in a regular interval and a wakeup notification interval isequal to a wakeup interval formed of an active duration and an inactiveduration; and broadcasting data to the neighboring nodes aftercompleting the request.

According to another aspect of the present invention, there is provideda node that operates in an energy saving mode in a network, the nodeincluding: a receiver receiving a wakeup notification from a first nodein the network; and a transmitter sending a wakeup notification in thesame interval as the first node and transmitting data in an activeduration estimated based on the wakeup notification received from thefirst node, wherein the wakeup notification interval is equal to awakeup interval formed of the active duration and an inactive durationand wakeup interval start times of nodes of the network are notsynchronized with each other.

Also, the transmitter transmits data within the active duration when theestimated active duration of the first node is greater than data sizeand requests an extension of the active duration to the first node whenthe active duration is less than the data size, and transmits datawithin the extended active duration after receiving a response to theextension of the active duration from the first node.

Also, the receiver receives a request for an extension of the activeduration from a second node and receives data in the extended activeduration from the second node, wherein the transmitter transmits aresponse to the extension of the activation duration to the second node.

The extended active duration returns to an initial active duration whendata transmission is completed.

According to another aspect of the present invention, there is provideda computer readable recording medium having embodied thereon a computerprogram for executing the energy saving method performed in a node in awireless network.

BRIEF DESCRIPTION OF THE DRAWINGS

The above and other features and advantages of the present inventionwill become more apparent by describing in detail exemplary embodimentsthereof with reference to the attached drawings in which:

FIG. 1 is a diagram illustrating a time structure according to anembodiment of the present invention;

FIGS. 2 and 3 are diagrams for explaining energy saving for mesh layer(ESM) unicasting, according to an embodiment of the present invention;

FIG. 4 is a diagram for explaining ESM broadcasting, according to anembodiment of the present invention;

FIG. 5 is a diagram illustrating a flow of a message used by a node inESM unicasting, according to an embodiment of the present invention;

FIG. 6 is a diagram illustrating a flow of a message used by a node inan ESM broadcast, according to an embodiment of the present invention;

FIG. 7 is a flowchart illustrating a method of transmitting ESM data,according to an embodiment of the present invention;

FIG. 8 is a diagram schematically illustrating a flow of a messagebetween nodes that operate in an ESM mode in a network, according to anembodiment of the present invention; and

FIGS. 9A and 9B are graphs showing active ratios compared through asimulation and experiment for data transmission/reception in aconventional Long Preamble Emulation (LPE) and LPEA (Long PreambleEmulation with Acknowledgement), and an ESM mode according to thepresent invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention will now be described more fully with reference tothe accompanying drawings, in which exemplary embodiments of theinvention are shown. In the drawings, like reference numerals denotelike elements, and the sizes and thicknesses of layers and regions areexaggerated for clarity. In addition, it will also be understood thatwhen some part “includes” some elements, other elements can be furtherincluded, instead of excluding them, as far as, there is no particularopposite description.

The present invention is directed to a method of minimizing power(energy) consumption when a node communicates with neighboring nodes ina mesh network. Unlike a conventional synchronous media access control(MAC), the present invention discloses an asynchronous MAC.

The synchronous communicating method generally used includes adjustingthe communication time of each node and a sink node is the main in thetime synchronization. However, in this method, communication isaccomplished only when the time is matched and it is difficult toaccurately match the time. Also, when such method is used, more power isconsumed by each node.

Thus, in the present invention, data is transmitted using asynchronousMAC. Command frames used in an energy saving for a mesh layer (ESM) ofthe present invention are Wakeup Notification WN, Extension REQuestEREQ, and Extension REPly EREP. The Wakeup Notification WN istransmitted to notify the status of a node while entering an activeduration. The Extension REQuest EREQ is transmitted to extend the activeduration of a reception node. The Extension REPly EREP is transmitted torespond to the Extension REQuest EREQ.

According to the present invention, a node receiving data (hereafter,reception node) periodically unicasts a wakeup message and a nodetransmitting data (hereafter, transmission node) broadcasts data. Inorder for the transmission node to transmit data, a wakeup period ofneighboring nodes may be analyzed and the transmission node may await awakeup notification from the neighboring nodes. When the transmissionnode receives a wakeup notification from the neighboring node, thetransmission node transmits a message. In addition, for low powerconsumption, when data to be transmitted does not exist, thetransmission node minimizes the active duration and when data to betransmitted exists, the transmission node transmits the ExtensionREQuest EREQ message for extending the active duration to the receptionnode. The reception node transmits the Extension REPly EREP forresponding to the Extension REQuest EREQ message to the transmissionnode.

The algorithm suggested in the present invention is accomplished by amethod of transmitting data and adjusting the active duration accordingto the amount of data.

In the present invention, ESM denotes an energy (power) saving modeusing an asynchronous time schedule. The time structure of a node(device) for ESM includes an active duration and an inactive duration. Acommand frame referred to as wakeup notification WN, is transmitted atthe beginning of the active duration. However, the active durations ofnodes are not synchronized.

In the data frame transmission, ESM is divided into two differenttransmission mechanisms: ESM unicasting and ESM broadcasting.

In ESM unicasting, a node uses a receiver-oriented mechanism. That is, atransmission node waits until a reception node enters the activeduration. Then, the transmission node transmits a data frame within theactive duration of the reception node. In ESM broadcasting, atransmission node uses a transmitter-oriented mechanism. That is, thetransmission node wakes up all neighboring nodes up by transmittingcommand frames for a time duration longer than a wakeup interval,thereby broadcasting the data frame.

FIG. 1 is a diagram illustrating a time structure according to anembodiment of the present invention.

Referring to FIG. 1, a time line of a node is divided into wakeupintervals. Each wakeup interval is then subdivided into an activeduration and an inactive duration. In the active duration, a nodenotifies that it is in the active duration by transmitting a wakeupnotification command. Then, the node waits for possible frametransmission. In the inactive duration, a node enters a low power(sleep) mode by turning off a receiver circuitry.

All nodes may have the same wakeup interval but may have differentactive durations. The active durations of the nodes are notsynchronized. Each node transmits a Wakeup Notification WN command atthe beginning of its active duration in order to notify neighboringnodes that it is in the active duration. When the Wakeup Notification WNcommand is transmitted, the wakeup interval and active duration of eachnode are notified to neighboring nodes. Thus, when a node receives theWakeup Notification WN command from a neighboring node, the node canknow the neighbor node is in the active duration and the time for theneighboring node to move to the sleep mode again.

FIGS. 2 and 3 are diagrams for explaining ESM unicasting, according toan embodiment of the present invention.

Referring to FIG. 2, when the active duration of a node A (Dev A) issufficient for data transmission, data is transmitted without adjustingthe active duration. The node A (Dev A) and a node B (Dev B) areneighboring nodes. A period of each node is equal to the wakeup intervalWI. The length of the wakeup interval WI is the same in both node A andnode B, but the beginning time thereof is different from each other. Thewakeup interval WI corresponds to the length between the Wakeupnotifications WN. Also, the wakeup interval WI is divided into theactive duration and the inactive duration. The active duration means anode sends and receives data. The inactive duration means a node doesnot send or receive data and remains in a sleep mode. Each node operatesin the minimum activation duration.

The Wakeup notification WN message is transmitted from each node. Whenthe data to be transmitted from the node B to the node A exists (Dataarrival), the node B waits until it receives the Wakeup notification WNmessage transmitted from the node A. When the node B receives the Wakeupnotification WN message from the node A, the node B transmits its datato the node A, since the node A is in an active state. Then, the node Bgoes back in a sleep (inactive) state. After the node A receives thedata, the node A can transmit a reception acknowledgement message ACK tothe node B.

Referring to FIG. 3, data is transmitted due to adjustment of the activeduration of the node A. The time structure of FIG. 3 is the same as thatillustrated in FIG. 2. When transmitting data from the node B (Dev B) tothe node A (Dev A), if it is determined that the transmission data islarge so that the data cannot be transmitted during the correspondingwakeup interval WI or when the active duration is too short so that datacannot be transmitted, the node B transmits the Extension REQuest EREQmessage, requesting to extend the activation duration, to the node A.When the node A receives the Extension REQuest EREQ message, the node Aextends its activation duration in proportion to the quantity of datatransmitted from the node B and transmits the Extension REPly EREPmessage including the extended time is transmitted to the node B, inresponse to the Extension REQuest EREQ message. The node B analyzes thereceived Extension REPly EREP message and transmits the message to thenode A during the corresponding active duration. Then, when the data iscompletely received, the node A returns the active duration to theoriginal minimum time. When data is received, the node A can transmitthe reception acknowledgement message ACK to the node B.

FIG. 4 is a diagram for explaining ESM broadcasting, according to anembodiment of the present invention.

Referring to FIG. 4, when data to be broadcasted exists (data arrival),a node C (Dev C) transmits the Extension REQuest EREQ message,requesting the active duration that is longer than the wakeup interval,to the neighboring nodes including the node A (Dev A) and the node B(Dev B) and broadcasts the data frame.

When the Extension REQuest EREQ message is received, the nodes A and Bextend their active durations and receive data.

Hereinafter, network operations of nodes using the ESM mode according tothe present are described.

For ESM mode operation, MAC primitives and PAN (personal area network)information base (PIB) variables to control the MAC layer and a meshlayer timer of 1 ms resolution as a reference clock are required.

ESM uses the MAC primitives such as MAC common part sub-layer(MCPS)-DATA, MCPS-PURGE, and MAC sub-layer management entity (MLME)-SET.MCPS-DATA is used to transmit or receive a command frame. MCPS-PURGE isused to remove a pended frame. MLME-SET is used to turn on/off thereceiver circuitry. The mesh layer turns off the receiver circuitry bysetting the macRxOnWhenIdle attribute in MAC PIB to FALSE. In additionto macRxOnWhenIdle, macMinBE and macMaxCSMABackoff need to be adjustedby the mesh layer in the ESM mode. For maximum energy saving, a promptresponse from the MAC is essential.

With regard to the mesh layer timer, the basic time unit is defined asmeshTimeUnit, which is equal to 1 ms. All timing parameters defined inESM use meshTimeUnit as a time unit and all operations are based on themesh layer timer.

The time structure in which the wakeup interval, which can be dividedinto the active duration and the inactive duration, is repeated can bedescribed using variables of meshWakeupOrder, meshActiveOrder,meshBaseActiveDuration. The MESH PIB attribute, meshWakeupOrder,describes the interval of the periodic wakeup. As an example, the valuesof meshWakeupOrder, WO, and the wakeup interval, WI, may be respectively0≦WO≦14 and WI=meshBaseActiveDuration*2WO ms. In this case, ifmeshWakeupOrder, WO is equal to 15, a node is always in the activeduration without transmitting the Wakeup notifications WNs. Therefore,the wakeup interval WI does not exist. The value meshActiveOrder isignored if meshWakeupOrder WO is equal to 15.

The MESH PIB attribute, meshActiveOrder, describes the length of theactive duration within the wakeup interval. The values ofmeshActiveOrder, AO, and the active duration, AD may be respectively0≦AO≦WO≦14 and AD=meshBaseActiveDuration*2AO ms.

Frame transmission of ESM is controlled by two variables,meshESMExpected and meshESMOn. meshESMExpected controls the transmissionmethod for start/join procedure.

If the value of meshESMExpected is TRUE, a node assumes that the networkis running in the ESM mode. Then, the node performs an ESMinitialization procedure. If the value meshESMExpected is FALSE, thenode performs an initialization procedure without considering ESM.

meshESMOn decides whether a node is in the ESM mode. After joining thenode to a network, the node transmits the Wakeup notifications WNsperiodically as defined in meshWakeupOrder if the node sets meshESMOn toTRUE. In addition, the node has an active duration and an inactiveduration as defined in meshActiveOrder. The active durations of nodesmay not be synchronized. If the value of meshESMOn is FALSE, all framesare transmitted without considering ESM.

ESM unicasting starts when MESH-DATA.request is issued. When meshESMOnis TRUE, the mesh sub-layer of a source node for transmitting data turnson the receiver circuitry and waits until it receives the Wakeupnotification WN from a destination node. Upon receiving the Wakeupnotification WN, the source node copies the value of meshActiveOrder inthe payload to meshDestActiveOrder, DO. After receiving the Wakeupnotification WN, the source node estimates that the destination node isin the active duration. Here, if the elapsed time of the mesh timer isshorter than the time determined by meshBaseActiveDuration*2DO, thesource node can estimate that the destination node is in the activeduration.

If the receiver of the destination node is in the active duration, atransmitter of the source node selects one of two transmission methodsto use according to the remaining active duration of the receiver. Whenthe receiver of the destination node is in the active duration and theremaining active duration of the receiver is long enough to transmit adata frame, the transmitter of the source node transmits the data frame.Here, MCPS-DATA.request is used. If the remaining active duration is notenough to transmit the long data frame, the transmitter of the sourcenode transmits an extension request EREQ command frame. In addition tothe condition, if the remaining active duration is not enough totransmit the long data frame, but longer than or equal to 3*meshTimeUnitor more, the transmitter of the source node transmits the extensionrequest EREQ command frame. The extension request EREQ command frame isa command frame to extend the active duration of the receiver of thedestination node. If the extension request EREQ command is a commandframe broadcasted at the MAC layer, the destination address is includedin the mesh payload.

The destination node, upon receiving the extension request EREQ, checksthe destination address. If the address matches its own mesh address or0xffff, the destination node extends the active duration by the timeduration defined in meshEREQTime. Then, the destination node transmitsthe extension reply, EREP, to the source node which in turn transmitsthe extension request EREQ, if the destination node has the address ofthe source node in the extension request EREQ. The transmitter of thesource node which transmits the extension request EREQ transmits a dataframe after receiving the extension resply, ERER.

As an example, a node may have the minimum active duration,meshBaseActiveDuration, without considering a long data frame in orderto achieve the maximum power saving. In order to transmit a long dataframe to the neighboring node, the transmitter of the node shouldrequests an active duration extension. Therefore, when the nodes set AOto 0, handshaking between the extension request EREQ and the extensionreply, EREP, can be mainly used.

If the MAC layer confirms with SUCCESS, the mesh layer transmitsMESH-DATA.confirm to the upper layer to inform SUCCESS.

FIG. 5 is a diagram illustrating a flow of a message used by a node inESM unicasting, according to an embodiment of the present invention.

Referring to FIG. 5, in node B having data to be transmitted, whenMESH_DATA.request is transmitted from the upper layer to the mesh layerof a transmitter, the mesh sub-layer (MCPS) turns on a receiver. Thenode A notifies the wakeup state (Wakeup Notification) to the node B andthe node B requests extension of the active duration to the node A(Extension Request). The node A responds to the node B in response tothe extension request. The node B transmits a data frame to the node Aand the node A transmits the acknowledgement message ACK to the node B.

In node A and node B, MCPS_DATA.request is transmitted from the meshlayer to the mesh sub-layer (MCPS) before transmitting the commandsframes such as wakeup notification, extension request, and extensionresponse and the data frames. Also, after transmitting the frames,MCPS_DATA.confirm is transmitted from the mesh sub-layer (MCPS) to themesh layer. In addition, in node A and node B, MCPS_DATA.indication istransmitted from the mesh sub-layer (MCPS) to the mesh layer afterreceiving the command frames.

After completing data frame transmission, when the ACK is transmittedfrom the node A to the node B, MCPS_DATA.confirm is transmitted from themesh sub-layer (MCPS) to the mesh layer and MESH_DATA.confirm istransmitted from the mesh layer to the mesh upper layer in the node B.

Meanwhile, when the node cannot receive the frames properly, each nodecan be operated as follows.

-   -   Lost Wakeup Notification WN: When a mesh layer of a node has a        frame to be transmitted, a receiver circuitry is turned on to        receive a Wakeup Notification WN of a destination node. At this        time, the node sets a timer that will expire after one wakeup        interval. If the timer expires before receiving the Wakeup        Notification WN, the node increases a ESM retry counter ERC by        one. If the ERC is less than meshMAXNumESMRetries, which is the        maximum number of retries, the mesh layer retries waiting the        Wakeup Notification WN again. If the ERC is equal to or greater        than meshMAXNumESMRetries, the mesh layer issues        MESH-DATA.confirm with status TRANSACTION_EXPIRED. Thus, data        transmission is completed and the ERC is initialized.    -   Lost Extension REPly EREP: When a mesh layer of a node transmits        an Extension REQuest EREQ, the node sets a timer to be expired        after meshEREPTime. If the timer expires before receiving the        Extension REPly EREP, the node increases ERC by one. If ERC is        less than meshMAXNumESMRetries, the mesh layer retries waiting a        next Wakeup Notification WN while turning on a receiver        circuitry, or by estimating the next Wakeup Notification WN time        and turning off the receiver circuitry until the estimated time.        If ERC is equal to or greater than meshMAXNumESMRetries, the        mesh layer issues MESH-DATA.confirm with status        CHANNEL_ACCESS_FAILURE.    -   Lost acknowledgement message ACK: If a mesh layer of a node        issues an MAC primitive for data transmission, the execution of        the primitive relies on a MAC layer operation. The mesh layer        should wait for the confirmation from the MAC layer. When        MCPS-DATA.confirm is returned by the MAC layer within an active        duration of the destination node receiver or extended active        duration of the receiver, the mesh layer issues        MESH-DATA.confirm with the same status to the upper layer.        However, if the active duration or extended active duration of a        destination node receiver is expired before receiving        confirmation from the MAC layer of the node, the mesh layer may        issue MCPS-PURGE.request to cancel the MAC layer transmission.        Otherwise, the MAC layer may issue MCPS-DATA.confirm with NO_ACK        status. In either case, the mesh layer increases ERC by one and        performs the retry procedure described above.    -   Lost data.: If a mesh layer of a destination node receiver        receives the Extension REQuest EREQ, the destination node sets a        timer to be expired after meshDATATime. If the timer expires        before receiving a data frame or another Extension REQuest EREQ        and the active duration of the destination node receiver ends,        the receiver may enter an inactive duration.

Broadcast transmission starts when MESH-DATA.request is issued forbroadcast, reliable broadcast, or multicast. If meshESMOn is TRUE, themesh sub-layer (MCPS) of the source node for transmitting data turns onits receiver and transmits the Extension REQuests EREQs for the timeduration longer than one wakeup interval. The destination address in themesh payload of Extension REQuest EREQ should be 0xffff. Then, atransmitter of the source node broadcasts a data frame.

If the transmitter receives Extension REQuest EREQ for broadcastingwhile transmitting Extension REQuests EREQs, the source node may stoptransmitting Extension REQuests EREQs and start receiving ExtensionREQuests EREQs to receive a data frame. At this time, the transmitterincreases an ESM retry counter ERC by one. If the ERC is equal to orgreater than meshMAXNumESMRetries, the mesh layer issuesMESH-DATA.confirm with status CHANNEL_ACCESS_FAILURE. After receivingthe data frame or encountering the Lost data condition, the transmitterof the source node retries ESM broadcasting. If the ERC is less thanmeshMAXNumESMRetries, the mesh layer retries ESM broadcasting.

If the MAC layer confirms with SUCCESS and the transmission is forbroadcast, the mesh layer transmits MESH-DATA.confirm to the upperlayer. In case the transmission is for reliable broadcast or multicast,the mesh layer follows the decision from its corresponding transmissionmethod.

FIG. 6 is a diagram illustrating a flow of a message used a node in ESMbroadcasting, according to an embodiment of the present invention.

Referring to FIG. 6, in node B having data to be transmitted, whenMESH_DATA.request is transmitted from the upper layer to the mesh layerof the transmitter, the mesh sub-layer (MCPS) turns on the receiver. Thenode B requests the neighbor nodes including the node A to extend theactive duration (Extension Request). The node A notifies the wakeupstate (Wakeup Notification) to the node B and the node B, whichcompletes the extension request, transmits data frames to the node A.

In the node A and the node B, MCPS_DATA.confirm is transmitted from themesh sub-layer (MCPS) to the mesh layer before transmitting the commandframes such as wakeup notification and extension request and data frame.After the transmission of the frames, MCPS_DATA.confirm is transmittedfrom the mesh sub-layer (MCPS) to the mesh layer. Also, in the node Aand the node B, after receiving the command frames, MCPS_DATA.indicationis transmitted from the mesh sub-layer (MCPS) to the mesh layer.

After completing data frame transmission, MCPS_DATA.confirm istransmitted from the mesh sub-layer to the mesh layer in the node B andMESH_DATA.confirm is transmitted from the mesh layer to the mesh upperlayer.

ESM may be started in two ways.

First, if meshESMExpected is set to FALSE, a node may start or join thenetwork without considering the ESM mode. After establishing a meshlayer address, the node can start ESM by setting the meshESMOn to TRUE.Since all associated procedures are performed before starting ESM, thebeacon request frame, the association request frame, and the datarequest frame can be transmitted without considering the ESM mode. Also,a beacon frame and an association response frame are received in thesame manner.

Second, if meshESMExpected is set to TRUE, a node assumes that othernodes are running in the ESM mode. The node selects one channel beforeissuing MLME-SCAN.request by MHSME-DISCOVER-MESH.request,MHSME-START-MESH-NETWORK.request, or HSME-START-MESH-DEVICE.request.Then, the node starts with the ESM broadcasting transmission method. Themesh layer transmits Extension REQuests EREQs during the time longerthan one wakeup interval and issues MLME_SCAN, request for one channel.Since the node does not have the mesh address yet, the source address ofthe Extension REQuest EREQ is set to, for example, 0xffff. ScanDurationis the time for transmitting the Extension REQuest EREQ and is definedfor the MAC layer timer. The value of ScanDuration should be interpretedas the approximated value in meshTimeUnit. If the mesh layer primitiverequires more than one channel, a temporary scan list can be made forthe channels. Then, each channel in the list is scanned in the samemanner. After the scan procedure, the node selects one channel to use.This may be achieved according to the mesh layer decision or theparameter in MHSME-START-MESH-DEVICE.request. The node starts a new meshor transmits an association request frame in the ESM unicastingtransmission method. The node waits for the Wakeup Notification WN andtransmits the request frame when the Wakeup Notification WN is received.If the node successfully receives an association response, the node setsmeshESMOn to TRUE. The node starts transmitting the Wakeup NotificationsWNs and schedules active and inactive durations. ScanDuration may beused to set a timer for the Wakeup Notification WN.

If a node that operates in the ESM mode receives the association requestframe, the node turns on the receiver circuitry during a predeterminedtime, for example, 2*macResponseWaitTime. For this exceptional period,the node transmits a Wakeup Notification WN. All data transmissions arethe same as in the ESM transmission methods.

FIG. 7 is a flowchart illustrating an ESM data transmission method,according to an embodiment of the present invention. Detaileddescription that is overlapped with above is omitted here.

Referring to FIG. 7, a source node of the ESM mode for transmitting datadetermines ESM unicasting or ESM broadcasting in operation S701. Whenthe source node determines ESM unicasting, the source node waits for andreceives the Wakeup Notification WN from a destination node for datatransmission in operation S702. When data to be transmitted exists, thesource node in the inactive duration is activated and waits for theWakeup Notification WN from the destination node. Nodes in the networknotify wakeups at a regular interval and the wakeup notificationinterval is same as the wakeup interval formed of the active durationand the inactive duration. The wakeup notification is performed at thebeginning of the active duration and the wakeup interval start times ofnodes are not synchronized with each other.

After receiving the Wakeup Notification WN, it is determined inoperation S703 whether the active duration of the destination node isenough to transmit data.

When the active duration is enough to transmit data, a data frame istransmitted into the active duration in operation S704. When the activeduration estimated based on the Wakeup Notification WN is greater thanthe data size (or data quantity), it can be determined that the activeduration is enough to transmit data.

When the active duration is not enough to transmit data, the ExtensionREQuest EREQ is transmitted for requesting an extension of the activeduration in operation S705. When the active duration estimated based onthe Wakeup Notification WN is less than a duration corresponding to thedata quantity, it can be determined that the active duration is notenough to transmit data.

Whether the Extension REPly EREP with respect to extending the activeduration is received from the destination node is determined inoperation S706.

When the Extension REPly EREP is received, the data frames aretransmitted within the active duration of the destination node inoperation S704 and when the Extension REPly EREP is not received, thenode waits to receive the Wakeup Notification WN again in operationS702. When data transmission is completed, the destination nodetransmits the acknowledgement ACK message and the extended activeduration returns to an initial active duration. When data transmissionis completed, the source node returns to the inactive duration.

Meanwhile, in case of ESM broadcasting, the source node broadcasts theExtension REQuest EREQ to the neighboring nodes in operation 707 and thedata frames are broadcasted after completing the request in operation708. The source node requests the neighboring nodes to extend the activeduration that is longer than the wakeup interval.

FIG. 8 is a diagram schematically illustrating a flow of a messagebetween nodes that operate in an ESM mode in the network, according toan embodiment of the present invention. Detailed description that isoverlapped with above is omitted here.

Referring to FIG. 8, each of nodes i and j respectively includesreceivers 10 and 50 and transmitters 30 and 80. In this embodiment, itcan be assumed that the node i is transmission node and the node j isreception node, and vice versa.

In node i, the receiver 10 receives the Wakeup Notification WN andExtension REPly EREP in response to Extension REQuest EREQ from thetransmitter 80 of node j. The transmitter 30 of node i transmits datawithin the active duration of node j estimated based on the WakeupNotification WN and transmits the Extension REQuest EREQ, if necessary,to the receiver 50 of node j. When the estimated active duration isgreater than data size (a duration corresponding to the data quantity),the transmitter 30 transmits data within the active duration and whenthe estimated active duration is less than data size (a durationcorresponding to the data quantity), the transmitter 30 transmits theExtension REQuest EREQ and transmits data to the extended activeduration after receiving the Extension REPly EREP from node j. In caseof ESM broadcasting, the transmitter 30 transmits the Extension REQuestEREQ to neighboring nodes and broadcast data when transmission of theExtension REQuest EREQ is completed. In this case, the Extension REQuestEREQ is the request for extending the active duration that is longerthan the wakeup interval.

In node j, the transmitter 80 periodically notifies the WakeupNotification WN in the same interval as other nodes to the receiver 10of node i, and the receiver 50 receives data within the active durationand receives data within the extended active duration, when theExtension REQuest EREQ is received from the transmitter of node i. Thetransmitter 80 transmits the Extension REPly EREP and the acknowledgmentACK message when data reception is completed to the receiver 10 of nodei. When data reception is completed, the extended active duration of thenode j returns to the original minimum active duration.

FIGS. 9A and 9B are graphs showing active ratios compared through asimulation and experiment for data transmission/reception in aconventional Long Preamble Emulation (LPE) and Long Preamble Emulationwith Acknowledgment (LPEA), and an ESM mode according to the presentinvention.

The data frame, the Wakeup Notification WN, EREQ/EREP, arrival rate, andthe experiment time used in the simulation and experiment arerespectively set to 50 byte, 28 byte, 27 byte, 0.01-0.00125 fr/s, and600-800 s. The active ratio is measured using turn on time/time spent.

The simulation result (anal) is indicated by a dotted line and theexperiment result (exp) is indicated by a solid line. In FIG. 9A andFIG. 9B, rb, which is probability of receiving broadcasting frame, isrespectively 0 and 0.00125.

Referring to FIGS. 9A and 9B, the active ratio of node in datatransmission/reception according to the ESM mode of the presentinvention is lower than that of in the conventional LPE and LPEA.Accordingly, the active duration is shortened in the present inventionin which the active duration flexibly varies so that energy saving ispossible.

According to the attribute of the sensor network, the frequency of datatransmission is not high or a large quantity of data is not transmitted.Instead, a small quantity of data is transmitted and the frequency ofdata transmission is low. That is, a sleep mode is maintained most ofthe time.

Therefore, in general, the active duration is minimized, thereby savingenergy. Only when a large quantity of data is transmitted, the activeduration is extended in the corresponding time and after thetransmission is completed, the extended active duration returns to theminimum active duration again, thereby minimizing power consumption.

The invention can also be embodied as computer readable codes on acomputer readable recording medium. The computer readable recordingmedium is any data storage device that can store data which can bethereafter read by a computer system. Examples of the computer readablerecording medium include read-only memory (ROM), random-access memory(RAM), CD-ROMs, magnetic tapes, floppy disks, optical data storagedevices, and carrier waves (such as data transmission through theInternet). The computer readable recording medium can also bedistributed over network coupled computer systems so that the computerreadable code is stored and executed in a distributed fashion. Also,functional programs, codes, and code segments for accomplishing thepresent invention can be easily construed by programmers skilled in theart to which the present invention pertains.

While the present invention has been particularly shown and describedwith reference to exemplary embodiments thereof, it will be understoodby those of ordinary skill in the art that various changes in form anddetails may be made therein without departing from the spirit and scopeof the present invention as defined by the following claims.

1. An energy saving method performed in a node in a wireless network,the method comprising: receiving a wakeup notification from adestination node by a source node having data to be transmitted, whereinthe source node is one of a plurality of nodes sending a wakeupnotification in a regular interval and a wakeup notification interval isequal to a wakeup interval formed of an active duration and an inactiveduration; and transmitting data within an active duration of thedestination node when the active duration estimated based on the wakeupnotification is greater than a data size.
 2. The method of claim 1,further comprising: requesting an extension of the active duration whenthe active duration is less than the data size; and transmitting data inthe extended active duration after receiving a response to the extensionof the active duration that is enough to receive data from thedestination node.
 3. The method of claim 2, wherein the extended activeduration of the destination node returns to an initial active durationwhen data transmission is completed.
 4. The method of claim 1, whereinthe wakeup notification is transmitted at the beginning of the activeduration and wakeup interval start times of the nodes are notsynchronized with each other.
 5. The method of claim 1, wherein thesource node in the inactive duration is active when data to betransmitted exists, so as to wait for the wakeup notification from thedestination node.
 6. The method of claim 1, wherein the source nodereturns to the inactive duration when data transmission is completed. 7.An energy saving method performed in node in a wireless network, themethod comprising: requesting extension of an active duration toneighboring nodes by a source node having data to be transmitted,wherein the source node is one of a plurality of nodes sending a wakeupnotification in a regular interval and a wakeup notification interval isequal to a wakeup interval formed of an active duration and an inactiveduration; and broadcasting data to the neighboring nodes aftercompleting the request.
 8. The method of claim 7, wherein the sourcenode requests an extension of the active duration that is longer thanthe wakeup interval.
 9. A node that operates in an energy saving mode ina network, the node comprising: a receiver receiving a wakeupnotification from a first node in the network; and a transmitter sendinga wakeup notification in the same interval as the first node andtransmitting data in an active duration estimated based on the wakeupnotification received from the first node, wherein the wakeupnotification interval is equal to a wakeup interval formed of the activeduration and an inactive duration and wakeup interval start times ofnodes of the network are not synchronized with each other.
 10. The nodeof claim 9, wherein the transmitter transmits data within the activeduration when the estimated active duration of the first node is greaterthan data size and requests an extension of the active duration to thefirst node when the active duration is less than the data size, andtransmits data within the extended active duration after receiving aresponse to the extension of the active duration from the first node.11. The node of claim 9, wherein the receiver receives a request for anextension of the active duration from a second node and receives data inthe extended active duration from the second node, wherein thetransmitter transmits a response to the extension of the activationduration to the second node.
 12. The node of claim 11, wherein theextended active duration returns to an initial active duration when datatransmission is completed.